1. Field of the Invention
The present invention relates to a backup power supply arranged in an apparatus.
2. Prior Art
Conventionally, in an apparatus operating under connection to a commercial AC power supply and in a computer suffering damage such as data loss if the commercial AC power supply fails, an uninterruptible power supply (UPS) is installed externally and a countermeasure for service interruption is taken. The UPS externally installed generally uses a fixed inverter power supply system. The fixed inverter power supply system UPS is free of the power supply switching operation at the time of service interruption and high in stability of the power supply. However, there are many series stages of converters through which the current passes during the period from the commercial AC power supply to the load, so that the power conversion efficiency is made lower and realization of power conservation is difficult.
On the other hand, a backup power supply that a secondary battery and a charge-discharge circuit for it are internally loaded and an external UPS is not required is proposed. As an example thereof, “UPS built-in power supply” in Japanese application patent laid-open publication No. Hei 09-322433 may be cited. The constitution of a conventional backup power supply is shown in FIG. 10. A commercial power supply 1 is connected to an AC-DC converter 3 and a charge circuit 8 installed in an information processor 2 and a secondary battery 4 and the input side of a DC-DC converter 7 are connected to the output side of the charge circuit 8. Further, the output side of the DC-DC converter 7 and the output side of the AC-DC converter 3 are connected to each other and connected to a load 6. Further, a balance control circuit 9 is connected between the AC-DC converter 3 and the DC-DC converter 7.
The operation of this circuit is shown in FIG. 11. (a) shows the stationary state and the commercial AC power supply 1 supplies 90% of the power necessary to the load via the AC-DC converter 3. Further, the charge circuit 8 supplies the residual 10% of power to the load via the DC-DC converter 7. Furthermore, the secondary battery 4 is charged via the charge circuit 8. On the other hand, (b) shows the operation during service interruption, and since the commercial power supply 1 fails, the charge circuit 8 and the AC-DC converter cannot operate, though the secondary battery 4 supplies all 100% of power necessary to the load via the DC-DC converter 7.
The aforementioned conventional backup power supply requires three converters such as the AC-DC converter, DC-DC converter, and charge circuit, so that problems arise that the price is high and the volume of the power unit is large.
Further, in this power supply system, the charge circuit operates always in the stationary state and a fixed voltage is applied to the secondary battery. However, when a secondary battery of high energy density such as a Ni—MH secondary battery or a Li ion secondary battery is used, to prevent overcharge, it is necessary to stop the charge circuit when the battery enters the full charge state. However, when the charge circuit is stopped by the aforementioned operation method, a problem arises that the DC-DC converter cannot supply 10% of power.
On the other hand, separately from this, a problem of the power supply capacity for load changes is imposed. This problem is that for example, in a load such as a hard disk device, a current 2 or 3 times of the normal load current flows at start and seek time. The rated capacity of the AC-DC converter is designed in accordance with the peak load time, so that the capacity of the AC-DC converter is increased and problems of high cost and difficulty in reduction of the capacity of power unit are imposed.